The Time Paradox (Artemis Fowl, Book 6)

After his three year absence, Artemis Fowl, big brother and former criminal mastermind, is faced with finding a cure for his sick mother by going back in time and stopping his former self from killing the animal that will cure his mother.

Long Synopsis:

Artemis's mother has contracted a deadly disease--and the only cure lies in the brain fluid of African lemurs. Unfortunately, Artemis himself was responsible for making the lemurs extinct five years ago. Now he must enlist the aid of his fairy friends to travel back in time and save them. Not only that, but he must face his deadliest foe yet...his younger self.